<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="lib/vue-2.4.0.js"></script>
    <script src="lib/vue-router-3.0.1.js"></script>
</head>
<body>
<div id="app">
    <input @keyup="getFullName" type="text" v-model="firstname">+
    <input @keyup="getFullName" type="text" v-model="lastname">=
    <input type="text" v-model="fullname">
</div>
<script>
    var vm=new Vue({
        el:"#app",
        data:{
            firstname:'',
            lastname:'',
            fullname:'',
        },
        methods:{
            getFullName(){
                this.fullname=this.firstname+'---'+this.lastname
            }
        }
    })
</script>
</body>
</html>